Semipermeable membranes can be used for separations or purifications by permitting the flow of a liquid through the membrane while blocking the flow of some or all of the substances that are dispersed or dissolved in the liquid. Substances that are blocked from passing through the membrane are referred to as “retained.” Semipermeable membranes can be characterized by the size of the substances that are separated from the liquid. Generally, ultrafiltration membranes retain substances having a size from about 0.002 μm to about 0.05 μm. Microfiltration membranes are typically used to retain substances larger than this, and nanofiltration membranes are typically used to retain substances smaller than this. Membranes for ultrafiltration or nanofiltration are typically made with precise filtration properties so as to retain substances only above a given molecular weight.
Ultrafiltration membranes are especially useful in cross-flow filtration, in which an unfiltered feed liquid flows substantially parallel to the surface of the membrane. As the feed liquid passes across the surface of the membrane, the permeate liquid will pass through the membrane, and the remaining concentrated feed liquid will continue to flow in the same direction as the unfiltered feed liquid. Unlike dead-end filtration processes, in which the flow of unfiltered liquid is perpendicular to the surface of the membrane, the membrane in a cross-flow filtration system is not as susceptible to fouling due to the accumulation of retained substances on the membrane surface. As a result, cross-flow filter membranes typically have a longer service life than dead-end membranes. Moreover, while in service, cross-flow filtration systems typically exhibit more consistent separation capabilities.
A material that has proven useful for making ultrafiltration membranes, including cross-flow filtration membranes, is poly(vinylidene fluoride) (PVDF). This polymer has desirable mechanical properties and also exhibits good resistance to chemical degradation. In addition, PVDF can be formed into membranes having controlled porosity, allowing for good control over the separation and retention characteristics of the membrane. One potential drawback to PVDF is its hydrophobic nature, which makes it difficult to be wetted with aqueous liquids. Most liquid separations involve aqueous liquids, and a hydrophobic membrane will tend to reduce the flow of the liquid through the membrane. In addition, biological substances such as proteins can tend to adhere to hydrophobic surfaces, leading to fouling of the membrane.
A variety of approaches have been described for modifying PVDF membranes to make them hydrophilic rather than hydrophobic. Coating a fully formed PVDF membrane with a hydrophilic polymer can result in a hydrophilic surface. However, this approach greatly increases the complexity of the manufacturing process and can result in a loss of control over the porosity and the retention properties of the membrane. The coating layer can also be degraded by the liquid being filtered, causing the properties of the membrane to change over time.
A slightly simpler approach involves forming the membrane from a mixture of PVDF and a water-soluble polymer such as poly(vinyl pyrrolidone) (PVP). The effect of the PVP additive, however, is not strong enough to provide a permanently hydrophilic membrane that can be dried and re-wetted. Thus, these membranes are usually dried in the presence of a wetting agent to facilitate the formation of a hydrophilic surface. The temporary nature of such a hydrophilic surface also requires the membrane to be maintained in a wetted state. If the membrane is allowed to dry out, it can be difficult or impossible to restore the hydrophilic surface. This increases the complexity of the packaging and shipping of the membrane or of a filter containing the membrane, and also requires the user to monitor the membrane once it is in use. Additional preparation steps, such as flushing the wetting agent from the membrane prior to use, may also be required of the user. Another drawback of this method is that the water-soluble polymer tends to leach out of the membrane fairly quickly, resulting in a short service life.
It is desirable to provide a semipermeable membrane that has a permanent hydrophilic surface, yet can be produced, stored, and used simply and efficiently. A system for producing such a hydrophilic semipermeable membrane would desirably allow for precise control over the retention and flow characteristics of the membrane. A controlled hydrophilic membrane potentially could be useful in ultrafiltration systems, including cross-flow filtration.